The first positive light leaf spot symptoms of the 2019/2020 season have been identified in growing crops of oilseed rape by Bayer’s SpotCheck initiative.

This important disease saw it being first identified this growing season by SpotCheck on October 25, in Derbyshire and Angus – with other samples later positively identified in the Scottish Highlands, Oxfordshire, Yorkshire and Norfolk. The results suggest that this season there may be a lot of latent infection in the crop, which is expected to further develop unless temperatures fall sharply, as they did in many parts of Scotland this week.

These results came as no surprise to Philip Walker, an arable plant pathologist with ADAS, who cited above average rainfall in September and October as being a key driver of infection. “Moisture and particularly leaf wetness is important to light leaf spot infection occurring. For infection to occur, there needs to be a minimum of six hours of leaf wetness and as those hours of leaf wetness increase, so too does the likelihood of infection.

“Last season, initial infections were almost a month later [end of November]. Finding samples of light leaf spot so much earlier this season suggests there is a lot of latent infection in the crop, and we will see more and more symptoms developing if temperatures don’t drop in November and December,” he added.

Ella Crawford, Bayer’s commercial technical manager for Suffolk, said the results were of particular note for earlier drilled crops, which are at greater risk. “Oilseed rape crops remain a mixed bag, with many areas experiencing extensive flea beetle damage and being written off before incurring further input costs,” she said.

“However, there are a number of more forward, lush crops – particularly those drilled early August. These will be at higher risk of light leaf spot infection because the crop is exposed to ascospores for a longer period of time.”

She reminded growers that phoma is more damaging on smaller and backward crops. “These will need to be treated as soon as the 10% threshold is met. However, as conditions have also been conducive to light leaf spot infection, growers should consider a product active against both phoma and light leaf spot, such as Proline (prothioconazole) at 0.46 l/ha.”